Mr Gates said he and his wife decided to put their money into vaccines after seeing the need at first hand during their recent travels.

'No link to trial'

The gift comes as Microsoft is embroiled in a complex court case over its alleged attempts to monopolise parts of the software industry.

But Mr Gates said there was no link between the legal proceedings and the timing of his donation.

"If there are any two things that are more unrelated, I'd say this has got to be it," he said.

"Although Microsoft has provided the wealth to do this, I don't see this as being related in any way."

The initiative will be administered by the Seattle based Programme for Appropriate Technology in Health, in conjunction with the World Health Organisation (WHO), Unicef, the World Bank and the pharmaceutical industry.

'Extraordinary gift'

The WHO says it is the largest single gift ever made to promote children's vaccinations.

Carol Bellamy of Unicef said the problem with many of the new vaccines is that there has not been the means to get them to the developing countries where they are most needed.

"Gates' money is a way of opening the door," she said.

"It will not solve all the problems, but it will challenge others."

She added that it would push the immunisation programme along "not just by an inch, but by a foot.

"It really is an extraordinary gift."

One of the richest men in the world, Mr Gates's wealth would cover the whole of Manhattan if laid out in $500 bills, according to one estimate.

But he is not the only high profile US entrepreneur to give in recent times.

In a television interview at the time, he said many highly wealthy individuals in America "won't loosen up their wads" because they are obsessed with ranking high on the Forbes magazine list of the 400 richest Americans.

He said: "I was appalled how little some of them were giving away.

"And you're not happy if you're a Scrooge."

WATCH/LISTEN

ON THIS STORY

Audio
Bill Gates tells the BBC about his $100m gift and the Microsoft court case

Video
The BBC's John McIntyre: "The Gates join the ranks of big time philanthropists"